Allah is the rewarder the fasting ( As-Siyam)

Abu Hurairah (May Allah be pleased with him) reported:
The Messenger of Allah (pbuh) said, "Allah the Exalted and Majestic said: 'Every act of the son of Adam is for him, except As-Siyam (the fasting) which is (exclusively) for Me, and I will reward him for it.' Fasting is a shield. When anyone of you is observing fast, he should neither indulge in obscene language nor should he raise his voice; and if anyone reviles him or tries to quarrel with him, he should say: 'I am fasting.' By Him in Whose Hand the soul of Muhammad is, the breath of one observing Saum is sweeter to Allah than the fragrance of musk. The one who fasts, experiences two joys: he feels pleasure when he breaks the fast. He is joyful by virtue of his fast when he meets his Rubb."

[Al-Bukhari and Muslim].

In a narration by Al-Bukhari, the Messenger of Allah (pbuh) said, "Allah says: '(The person observing Saum) has abstained from food and drink, and sexual pleasures for My sake; fasting is for Me, and I will bestow its reward. Every good deed has ten times its reward'."

In a narration by Muslim, the Messenger of Allah (pbuh) said, "The reward of every (good) deed of a person is multiplied from ten to seven hundred times. Allah (SWT) says: 'The reward of observing Saum is different from the reward of other good deeds; Saum is for Me, and I Alone will give its reward. The person observing Saum abstains from food and drink only for My sake.' The fasting person has two joyous occasions, one at the time of breaking his fast, and the other at the time of meeting his Rubb. Surely, the breath of one observing Saum is better smelling to Allah than the fragrance of musk."